Project Summary/Abstract The proposed research uses bilingualism as a means to study general principles underlying human language and cognition, as well as an end in itself to understand how the rapidly growing bilingual segment of the U.S. population (and the majority of the world's population) processes language. The objective of the proposed research is to examine the consequences of bilingualism for language (Aim 1), cognition (Aim 2), and the brain (Aim 3). The methods employed include eye-tracking, electroencephalography, computational modeling, and cognitive and linguistic testing of bilinguals and monolinguals. Our previous research has shown that bilinguals co-activate both languages in parallel during spoken language comprehension when input overlaps in phonological form across languages. Study 1 aims to reveal covert co-activation of the non-target language during spoken comprehension when there is no overlap in input form, via multi-step cascading activation from the co-activated translation equivalent to phonologically overlapping items in the non-target language. Study 2 will examine the roles of top-down, lateral, and bottom-up mechanisms during co-activation in bilingual spoken language processing. Studies 3-5 will show that system changes as a result of experience with two interacting languages are not limited to linguistic processing, but also extend to non-linguistic cognitive processes such as visual search. This work will show that eye movements during visual search are influenced by co-activation of the two languages even when no linguistic information is present, thereby demonstrating that language experience changes visual search. Study 6 will use EEG to look at the neural signature and timecourse of language co-activation and control during bilingual spoken language comprehension. Theoretically, the proposed studies contribute to understanding how experience, particularly experience with two languages, reconfigures cognitive architecture and changes linguistic, cognitive, and neural function. This research illustrates the plasticity of the human brain as it adapts to accommodate multiple languages and provides insight into the relationship between language and cognition from the unique vantage point of bilingualism. Addressing broader societal needs, this work has practical implications for the large segment of the American population speaking a language other than English at home, for whom clinical and educational outcomes could be improved by developing interventions that capitalize on the interaction between the two languages, for example by using form-overlapping items (phonological cohorts, cognates) to facilitate co- activation of the two languages during treatment and learning. Health services depend on accurate models of cognitive, linguistic, and neural function, and the proposed research contributes to the development of such accounts for people whose systems are changed by bilingualism, so that the benefits of scientific knowledge are not limited to a subset of the population and extend to linguistically diverse groups.